1. Field of the Invention
An object of the present invention is a safety device for an integrated circuit. It is more particularly designed for the field of chip cards where it proves to be necessary to prevent fraudulent behaviour.
2. Description of the Prior Art
There already exist known protection systems of the electrostatic type, wherein protection circuits shield the integrated circuits against overvoltages. These overvoltages are, for example, caused when chip cards are used, through friction between the support of the chip cards and the reading machines, or between this support and the users' clothes. These protection systems are based on a simple principle. It includes interposing a semiconductor junction in parallel with each access terminal of the integrated circuit. This semiconductor junction is triggered for a pre-determined level of a voltage applied to this terminal with respect to another terminal (generally connected to the substrate). It has been observed, however, that in certain circumstances the substrate currents that result from making this junction conductive tend to modify the working of the integrated circuit.
Today, the greatest concern of circuit designers, especially for chip cards, is safety against possibly fraudulent individuals. This safety can be obtained in two ways. It can be obtained, on the one hand, through the software and, on the other, through the hardware (the circuits). No safety system is totally effective. Wired logic circuits (wherein there is no programming) are made safe solely by the hardware, while microprocessor circuits use both types of safety systems. In particular, in chip cards, both these types of safety systems are used. Consequently, there is always a hardware type of safety in the cards.
In the most elementary type of fraudulent activity with a card, the behavior of the circuit is modified by using only those terminals of the circuit that are accessible by a reader. This simple type of fraudulent behavior does not require many means. It is generally easy to implement. If the circuit is designed properly, there is no possibility of logic-type fraudulent behavior wherein abnormal logic signals are applied to the different terminals. But, on the other hand, there is nothing to prevent the fraudulent person from applying voltage levels, quite beyond the specifications of the circuit, to its terminals. These voltage levels could cause major substrate currents which themselves lead to programming or deprogramming operations that are harmful to the circuit.
In the integrated circuits, the terminals are all protected against electrostatic discharges. These systems of protection consist chiefly of diodes, bipolar transistors and resistors. They enable the removal of the charges present at the terminals once the voltage at this terminal has gone beyond a certain value. The charges to be removed are injected into the substrate of the circuit. They are then removed by a ground terminal to which this substrate is connected, at long intervals, by a ground connection. The electrostatic type of protection is aimed solely at preventing the junctions contained in the integrated circuit from being subjected to voltages that could lead them to breakdown. A situation is thus envisaged where a fraudulent individual could try to subject some of the connection terminals of the integrated circuit to overvoltages that make these electrostatic protection circuits operate. When these electrostatic protection circuits are in operation, high substrate currents develop in the substrate. These currents may possibly alter the logic behavior of the circuit. For, it can be shown that the substrate current may easily reach one ampere. Beyond one ampere, the wires connecting the integrated circuit to the support of the chip card that contains it melt, and the circuit becomes unusable. However, just below this value of one ampere, and with a resistance of one ohm between the terminal subjected to the overvoltage and the ground terminal, a drop in potential of one volt is obtained. In this case, the voltage of the substrate and of the ground connection rises locally to one volt with respect to the ground terminal itself. If the signal produced locally at this position of the circuit is transmitted to another part of the circuit, which would not be affected by this current, the theoretical input level of zero is considered to be carried to one volt and it may be detected as a logic 1. Such dysfunctioning in circuits has already been observed.
Furthermore an unwanted thyristor triggering phenomenon, called a "latch-up" phenomenon, may occur in CMOS type integrated circuit technologies. Latch-up problems such as this are commonly encountered when voltages, negative with respect to the potential imposed on the ground terminal, are injected into the connection terminals of the integrated circuit. This latch-up phenomenon may lead a circuit to work abnormally and, in certain cases, may even be difficult to detect. The result thereof is that, even without any fraudulent intent on the user's part, situations may be faced where dysfunctioning takes place.